1. Field of the Invention
This invention relates to a water-soluble hotmelt adhesive, to a process for producing it and to its use in the textile, packaging and paper industry. The invention also relates to at least two-ply paper laminates and moisture-tackifiable papers and textile laminates which contain the water-soluble hotmelt adhesive according to the invention as well as to processes for producing these two-ply laminates, moisture-tackifiable papers and textile laminates.
2. Discussion of the Related Art
Hotmelt adhesives are adhesives which are solid at room temperature and at least substantially water- and solvent-free, e.g., the amount of water/solvent is less than 5% by weight. Hotmelt adhesives are applied from the melt to the substrates to be bonded and set due to solidification on cooling.
Base polymers substantially determine the properties important for the hotmelt adhesives such as adhesion, tenacity and temperature behaviour.
The following polymers are conventionally used as base polymers in hotmelt adhesives: thermoplastic polymers such as block copolymers, e.g., styrene-butadiene, styrene-butadiene-styrene, styrene-isoprene-styrene, styrene-ethylene butylenes-styrene, styrene-ethylene propylene-styrene; ethylene vinyl acetate polymers, other ethylene esters and copolymers, e.g., ethylene with acrylate, ethylene n-butyl acrylate and ethylene acrylic acid; polyolefins such as polyethylene and polypropylene, e.g., amorphous propylene α-olefins (APAO); polyvinylacetate (PVAc) and PVAc copolymers, polyacrylates, polyamides, polyesters, polyvinylalcohols (PVA) and PVA copolymers, polyurethanes, polystyrenes, polyepoxides, copolymers of vinyl monomers and polyalkylene oxide polymers, aldehydes, which contain resins such as phenol aldehyde, urea aldehyde, melamine aldehyde and the like.
Pressure-sensitive as well as non-blocking adhesive systems that can be granulated are commercially available.
Hotmelt adhesives are widely used in the packaging and paper industry for sealing and closing cartons, as moisture-tackifiable adhesives on stamps and envelopes or for producing at least two-ply paper laminates for use as hygiene paper. These hotmelt adhesives have to fulfill many requirements in that branch of industry: The hotmelt adhesives therefore have to have a middle to long open time (defined as the time interval between adhesive application and the fixing of parts to be bonded), but after the fixation of the parts to be bonded they need to set fast enough to guarantee a high-quality bond, in particular on fast-running packaging, paper processing or labelling machines. Beside the speed of setting, viscosity is another important criterion for the selection of the hotmelt adhesive. For mechanical processing, especially for uniform hotmelt adhesive application, the viscosity has to be sufficiently low at the respective application temperature. For bonding of packagings in the frozen-food area where packaged goods are stored at temperatures down to −35° C. low temperature flexibility must be provided. That means that at low temperatures, high tenacity of the adhesive bonding has to exist and the adhesive should not get brittle or break. On the other hand, warm or hot ready-prepared food or beverages are often packaged, in which case appropriately high stability of the adhesive is required. Sufficiently high stability means that the adhesive does not soften or flow under the influence of elevated temperature and therefore cause detachment and/or cause the bonded parts to shift against each other. Furthermore, especially for the food industry, substantially odor-free or low-odor hotmelt adhesives are required. Components which are used in corresponding hotmelt adhesives have to be licensed for use in the food industry. In addition it has to be remembered that very different substrates are bonded to each other, such as paper, cardboard, cartons coated with polyolefins and many more. Sufficient adhesion of the hotmelt adhesive to all these different substrates must be present.
Another requirement for hotmelt adhesives, especially those used for bonding hygiene papers, is that they should be as soluble as possible in water. This requirement is due to the large amounts of recycled paper used in the production of hygiene papers, i.e. production waste must be able to be directly reused in the production of new paper layers. To this end, however, the adhesive between the individual layers has to be completely removed from a recycled fibre slurry. Water-insoluble or substantially water-insoluble adhesive constituents, which can pass into the paper web from the recycled fibrous material during the production of a new paper layer, can lead to serious production downtimes. These adhesive residues normally form what are known as “sticky spots” in the recycled paper webs, i.e. tacky spots which cause the paper web to stick on rollers and similar guide elements for the paper web. This normally results in tearing of the paper webs and prolonged interruptions in production. Accordingly, not only must the bond itself be reversible by water, leading to a dispersion of the adhesive in water, but the adhesive itself must be completely soluble in water. This prevents any discharged adhesive residues from leading to tacky agglomerates.
Water-dispersible hotmelt adhesives are known. U.S. Pat. No. 4,129,539 discloses a hotmelt adhesive, which is dispersible in cold water and based on a copolymer of vinyl acetate and crotonic acid, a colophonium ester of dipentaerythritol and an aliphatic amino alcohol. A disadvantage of the disclosed adhesive system is that it does not form a clear aqueous solution. In addition, the adhesive strength is often insufficient for bonding paper laminates.
A process for producing at least two-ply paper laminates or moisture-tackifying materials is known from WO 99/59808, in which a water-soluble hotmelt adhesive based on a polyalkylene glycol with a molecular weight (M n) of 1,000 to 100,000 or a nonionic polyurethane with a molecular weight (Mn) of at least 2,000 or a polyester with a molecular weight of at least 3,000 is disclosed. The bonding strength of these compositions is often also insufficient.
Accordingly, an object of the present invention was to provide a hotmelt adhesive for the packaging and paper industry which can be applied as a melt, establishes a strong, permanent and rapid bond, even in the smallest quantities (less than 0.3 g/m2), after assembly of paper and, in addition, is completely soluble in water, i.e., miscible with water in any quantity. The hotmelt adhesive must also be suitable for fast-running manufacturing processes on packaging and labelling machines and in hygiene paper production and processing.
To produce textile laminates, textile fabrics, leather or synthetic leather are sewed together. Conventional sewing techniques include tufting, crocheting or knitting.
The congruent sewing of cut textile pieces is a problem here. Depending on the type and size of material and the manufacturing conditions selected, the superimposed cut textile pieces can slip, and this leads to production waste. The industry is looking for economical solutions for the congruent fixing of the cut textile pieces prior to sewing, without adversely affecting the material properties of the sewn textile laminate.
Starting from this prior art, a further object was to prepare a hotmelt adhesive for the production of textile laminates.